The present invention relates to a pressure sensor unit and, more particularly, to a pressure sensor unit that is encapsulated in a fluorinated polymer-containing material for protection to acquire an extended endurance span.
For the purpose of protecting a semiconductor device, such as a pressure sensor, against electric, mechanical or/and chemical impacts from outside, it has so far been known to install the semiconductor device in a housing and fill the vacant part of the housing with a gel material having excellent characteristics, including high electric insulation and strong resistances to thermal impact, vibration and both high and low temperatures. The gel materials prevailingly used for sealing up the housing are silicone resins, but the silicone resin gel has drawbacks of swelling in solvents, such as toluene, gasoline and gasohol, having high moisture permeability in spite of good water resistance and being subject to deterioration from chemicals, such as a strong acid and a strong alkali.
In the field of requiring gasoline resistance, water resistance and chemical resistance in addition to the aforementioned characteristics, it is therefore indispensable to use a fluorinated polymer-containing gel material having all the required characteristics. For instance, the pressure sensor unit used for pressure detection of fuel gas or exhaust gas of the engine in a car undergoes not only changes in pressure and temperature but also exposure to gasoline vapor, water vapor and acidic exhaust gases, so that the gel materials of fluorinated polymer-containing type are used as a suitable encapsulant for protecting the unit from the foregoing external impacts (Tokkai Hei 11-116685, and the term xe2x80x9cTokkaixe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent application).
As an example of the material forming such a fluorinated polymer-containing gel encapsulant, the curable perfluoro-polyether resin composition has so far been proposed which is constituted basically of (A) straight-chain fluoropolyether compounds containing an alkenyl group in each molecule and having a perfluoropolyether structure in each main chain, (B) fluorine-containing organic silicone compounds containing in each molecule hydrogen atoms bonded to silicon atoms and (C) a platinum group compound.
However, when the pressure sensor unit is encapsulated in only the gelled perfluoropolyether resin composition as mentioned above and put in an environment that the unit undergoes repeated changes in temperature and pressure in the atmosphere of the aforementioned gases, the gas bubbles are produced in the gel, and the produced bubbles grow or move to generate cracks in the gel or cut the bonding wires. As a result, there occurs a problem that not only the protective function of the gel but also the sensory function of the unit itself is lost.
As a result of our intensive studies to solve the aforementioned problem, it has been found that the endurance span of a pressure senor unit can be lengthened by covering at least the upper part of terminals with a film of fluorinated polymer-containing adhesive prior to encapsulation in a gel hitherto employed for pressure senor units, thereby achieving the present invention.
Therefore, an object of the present invention is to provide a pressure sensor unit having sufficient durability even when the pressure sensor unit is used in an environment that it undergoes- changes in temperature and pressure, and besides, it is exposed to the atmosphere of gasoline vapor, water vapor and acidic exhaust gases.
The aforementioned object of the invention is attained with a pressure sensor unit comprising (i) a housing, (ii) pressure sensor members installed therein which include a mount, the bottom of which is fixed to the inner base of the housing by means of an adhesive, a sensor chip firmly bonded to the top of the mount, terminals disposed on the inner base of the housing and bonding wires connecting the sensor chip to their respective terminals, and (iii) a encapsulant filling the housing so that all the pressure sensor members are completely buried therein; with the terminals being covered with a film of fluorinated polymer-containing adhesive composition in at least an upper part thereof, and the encapsulant being a fluorinated polymer-containing gel material, wherein the composition as adhesive is similar in fluorinated polymer to the gel material.
In the present pressure sensor unit, the terminals are covered with a film of fluorinated polymer-containing adhesive composition in their respective upper parts, and further entirely buried under the gel encapsulant. Therefore, they are hard to undergo the influences of gasoline vapor, water vapor and acidic exhaust gases even when the unit is exposed to the atmosphere containing those vapors and gases. Further, even when the unit is put under a condition to suffer changes in pressure and temperature, the encapsulant is effectively prevented from producing gas bubbles and generating cracks; as a result, the pressure sensor buried in the encapsulant can have a substantial improvement in endurance span. Accordingly, the present unit is useful as a pressure sensor unit loaded in cars.